Americans marked the 40th anniversary of one of the most famous speeches ever uttered Thursday. It was the 1963 "I Have a Dream" speech by civil rights leader Dr. Martin Luther King Jr.

Forty years ago, Martin Luther King Jr. stood on the steps of the Lincoln Memorial in Washington and issued a battle cry that stirred the nation in the struggle for civil rights.

"I have a dream that my four little children will one day live in a nation where they will not be judged by the color of their skin but by the content of their character. I have a dream today!" he said.

On Thursday, hundreds of people linked arms and marched in Dr. King's hometown of Atlanta, Georgia, to commemorate the anniversary of his speech.

Dr. King's widow, Coretta Scott King, was among those who spoke to the crowd. "Martin's call to interracial brother and sisterhood has an enduring resonance because it speaks so eloquently to the longing for unity that resides in the hearts of all people of good will," she said.

In Washington, Dr. King's son, Martin Luther King III, said the speech 40 years ago awakened millions around the world to the plight of racial inequality. But he emphasized much more remains to be done in closing the economic gap between whites and minorities.

"How are we going to bring America back, in a real sense, not just our economy but America for what it should represent and become. I think that is the question that we must find a way to answer," he said.

Historians have long viewed Dr. King's 1963 speech as a turning point in the struggle for civil rights in the United States. Its appeal for moral justice came at a time when whites were resisting federal efforts to end racial segregation in the south.

"The King speech after a long day of speeches was kind of like a volcano that just erupted," said Douglas Brinkley, a University of New Orleans historian. "It was one of those moments in place and time that just sort of set the tone and tenor of the entire civil rights movement."

Professor Brinkley and other historians rank the speech as one of the greatest in American history.

The final lines of the speech remain as the enduring image of Martin Luther King's legacy. He said: "We will be able to speed up that day when all of God's children, black men and white men, Jews and Gentiles, Protestants and Catholics, will be able to join hands and sing in the words of the old Negro spiritual, 'free at last, free at last, thank God almighty we are free at last.'"